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E39 540i low compression

99K views 416 replies 31 participants last post by  JimLev 
#1 ·
Hi guys,

I'm in the middle of what I thought it would be a valve cover gasket replacement and a few other relatively simple maintenance jobs, but the deeper I go the worse it becomes. First I noticed leaks from the head gaskets (yet to be confirmed). Yesterday I measured cylinder compression and it turned out evenly low in all cylinders at about 130psi as opposed to 170-203psi per spec (I've seen a post from one guy who measured his to be at 205 in average!). It was late at night and I didn't get a chance to do a wet test. I left the battery charge overnight just in case and will retest it again today followed by the wet test.

I read that if the wet test significantly and evenly improves compression it means worn piston rings, is that true in most cases?

My car is 1998, but only has 80k miles on it, is it a bit early for the piston rings to go? In the time that I owned it (a little over a year) I added roughly 10k miles on it and it never overheated.

Unless I'm missing it, I don't see a procedure for piston ring replacement in Bentley (if it comes to it). I haven't checked TIS yet, not sure if it has it. Where can I find the procedure explained in detail?

Any thoughts or suggestions are always greatly appreciated! :)

Here's a video of one of the cylinder tests, does it sound normal?



Thank you, guys, for any input!
 
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#71 ·
I have found that lacquer thinner works better than carb cleaners for removing combustion chamber deposits. You will still have to use quite a bit of elbow grease. If it's really necessary, a large FLAT honing stone is easier to use than sandpaper on the gasket surfaces. The flatness of the stone will prevent low spots from forming, at least it would yield a flatter result than sandpaper.
 
#72 ·
Thinner is good. Acetone is another.

I would keep all stones, of any kind, far away from an open engine.

Ok on heads, etc IF far removed from the engine compartment.
 
#75 ·
I'm going to refer all people who overheat to this thread (so I will add references, as needed, to flesh out the story for everyone).

As a compression cross reference, on 2/22, this overheating thread:
- Overheating and water loss
Resulted, this week, in an engine compression test:
- E39 528i Engine removal/replacement

With the compression figures of:
  • Cylinder 1 Dry - 14 bar, Wet - 17 bar
  • Cylinder 2 Dry - 12 bar, Wet - 16.5 bar
  • Cylinder 3 Dry - 15 bar, Wet - 17 bar
  • Cylinder 4 Dry - 8-12 bar, Wet - 17 bar (i thought i'd found the problem when this was dry at 8 bar, but repeated it dry and it returned 12 bar the second and third time)
  • Cylinder 5 Dry - 14 bar, Wet - 16.5 bar
  • Cylinder 6 Dry - 12 bar, Wet - 17 bar
These were not the expected figures; but that's what they were.
(Details here.)
 
#76 · (Edited)
I've spent most of the weekend cleaning parts, now most parts that I removed are shiny and ready to go back in. Next I'm going to thoroughly clean the engine bay, then the block deck, then the heads. When the heads are clean, I'm going to take them apart and clean/relap the valves. That obviously is a lot of cleaning, but I get a lot of satisfaction out if it, it is really nice to see everything nice and shiny.

I'm really happy that I've decided to go deeper then just the valve cover and the valley pan job. I've found that the gaskets on the timing covers, that used to be made of rubber at some distant point in the past, now appear to be made out of hard plastic and came off in little pieces. Also the oil pan gasket was leaking and was very hard as well, so were the valve cover gaskets. I've also found that not all bolts in different sets that I had to remove were equally torqued...
 
#78 ·
Thanks! Good idea on the oil pump bolts :)

I'm trying to think what else I can do while I'm there. Is there a way to evaluate the oil separator and the PCV valve? They both look fine, only PCV valve has some oily residue on its back, same as the inside of the intake manifold. I'm tempted to open PCV valve for cleaning/inspection, but it doesn't look easy to open and I'm afraid I'm just going to break the plastic tabs. Thinking of a safe way to open it...
 
#80 · (Edited)
540alex,

In the April issue of Popular Hot Rodding (PHR) they have an article called 'Power Tricks for any Engine'. It has 10 specific things you can do while building or re-building any 200 + hp V8 to p/u over 150 hp. All w/o replacing the internals, except for the cam and rings.

One of the items that you might be interested in doing, while you have complete access to your valves, is to have your valves back-cut (I quote this part of the article below) to p/u 5-15 hp by picking-up 5-10 cfm between 0.100 to 0.500" of lift. Those numbers are right from the article and a picture with the article. And, like I already said, apply to any V8.

Quoting right from the article... "This is one of those small tricks that is worth way more than the paltry sum it usually costs for this extra step. If you have the heads apart to lap the valves, or you have them at a shop, the easiest thing to do is mark the face of the valves with layout fluid or even a Sharpie, lap the valves in, then just have your machinist gives the valves a standard 30 degree back-cut up to the edge of the lap mark - simple and worth an average of 5-10 cfm, 5-15 hp. It's worth more power than most gasket-match jobs!".

For a typical 2 valve American V8 they estimate the total cost to be apprx $ 40. Of course that would be for 16 valves and you have 32. And a shop might charge more than $ 80 if you come in for the valve cut only. Or maybe not. If you have a lathe you can do it yourself.

For those that don't know, PHR has some great in-depth articles regarding American performance engine building, which can apply to any engine make, type, country of origin, etc. They also put out another magazine called Engine Masters that is in addition to PHR. American or not, engines are engines.

Below is PHR's web site. I did a quick look and couldn't find the article, probably because it just came out in the current issue.

http://www.popularhotrodding.com/index.html
 
#81 ·
540alex,

...One of the items that you might be interested in doing, while you have complete access to your valves, is to have your valves back-cut (I quote the full article below) to p/u 5-15 hp by picking-up 5-10 cfm between 0.100 to 0.500" of lift. Those numbers are right from the article and a picture with the article. And, like I already said, apply to any V8.

Quoting right from the article... "This is one of those small tricks that is worth way more than the paltry sum it usually costs for this extra step. If you have the heads apart to lap the valves, or you have them at a shop, the easiest thing to do is mark the face of the valves with layout fluid or even a Sharpie, lap the valves in, then just have your machinist gives the valves a standard 30 degree back-cut up to the edge of the lap mark - simple and worth an average of 5-10 cfm, 5-15 hp. It's worth more power than most gasket-match jobs!".

http://www.popularhotrodding.com/index.html
When I was stationed in Germany in the late 60's, we raced a '67 Mini-Cooper that was prepared by a former Downton (works) mechanic. He ground down the backs of the valves and polished them to the point that they looked like delicate jewelry. This not only allowed tremendous airflow, it lightened the valve train enough that the racing redline in the pushrod, long-stroke 1275cc engine was 7600 rpm.
 
#82 · (Edited)
540alex,

An engine with enough miles to have low compression could also have less than specified closed and/or open spring loads on one or more valve springs. Please don't misread this. I'm not saying weak springs are the case of your low compression. I am saying that the miles that would cause your worn seats would probably cause some springs to be below spec loads. With your meticulous cleaning it would be a shame to reinstall even one weak spring. Even worse to not check them at all

I realize its easy to sit here and suggest things for you to do but I would be remiss to know this and not mention it.
 
#87 ·
When one subscribes to PHR they will also periodically (but not monthly) recieve 'Engine Masters' which is a gold mine of inside technical info, besides being great fun to read, if you are into the technical side of engines. Engine Masters is not available for subscription by itself. Its only via PHR subscription or on the rag racks.

www.enginemasters.com will take you to PHR dot com.
 
#86 · (Edited)
540alex,

The spring load is important to prevent the valves from 'floating' at less that the engine's rpm limit. It is not about preventing the valves from leaking. Even a very weak spring will close a lapped and stationary valve and prevent it from leaking.

A spring load tester, the equipment and not the person, will read-out the lbs of force it takes to compress the spring at two different hts. One height is when the spring is installed and the valve is closed, and is called the seat load, and the other is when the valve is fully opened. The load at each of the two specified hts is important, though the open load figure is most important.

Any machine shop that works on heads will (should) have one.

The Bentley manual (page 116-56 thru 60) does not provide these specifications. It uses a down and dirty method, says to line up all the springs (on a very flat surface), place a straight edge across the top and replace any spring that is short. That is a quick (and cheap) method for a repair shop. That method will often not find a weak spring. A weakened spring can still retain full ht yet allow valve float. All the good performance books and magazines will tell you so. That is why a spring load tester should be used. If BMW will not supply height and load specs a machine shop with a load tester will still be able to find any weak springs.

Back-cutting will not affect long term reliablity. The valve stem is the weakest part of the valve, not the valve head. And like Bobdmac said it will lighten the valve, which will increase long term reliability, if it affects reliablity at all.

Sorry for the long post, but in my opinion its better to be clear and thorough than short.
 
#88 ·
The spring load is important to prevent the valves from 'floating' at less that the engine's rpm limit. It is not about preventing the valves from leaking. Even a very weak spring will close a lapped and stationary valve and prevent it from leaking.

A spring load tester, the equipment and not the person, will read-out the lbs of force it takes to compress the spring at two different hts. One height is when the spring is installed and the valve is closed, and is called the seat load, and the other is when the valve is fully opened. The load at each of the two specified hts is important, though the open load figure is most important.

....
franka,

I don't mind the long post at all, thanks a lot for taking the time to go into details. I'll have someone evaluate my springs when they are out (this weekend, I hope).

I read a couple of articles about back-cutting and (correct me if I'm wrong) it seems that it's mostly done to a "penny-on-a-stick" type of valves, not the tulip type that BMW uses.
 
#90 ·
Nice photo there. It clearly shows the additional 45 degree bevel of the back cut. The problem with it is that he comes one step closer to the seductive lure of porting and polishing the head, and who knows where that could lead?
 
#92 ·
The "power groove" idea sounds plausible, but I have two problems with it. First, I'd have to be shown before and after performance figures proving that it actually works. It seems to me that it could also cause additional turbulence that could obstruct the flow of the intake gases. Unfortunately, I no longer have a picture of them, but I can still see the those valves from that Mini Cooper that were actually ground down to a much thinner cross-section and polished to a mirror surface. That engine vaporized so much fuel that we'd get ice crystals forming on the outside of the intake manifold after a couple of hot practice laps.

Second, how much return do you get for how much cost? After a while, you reach a point of diminishing returns. In other words, it costs more and more for increasingly smaller incremental improvements.
 
#93 ·
King of hi tech racing engines are F1 engines and none of them use such valves.

I've seen them before but have never heard of any racing team or mfgr ever using them. And I've been reading on this stuff for longer than some here have been alive.
 
#94 ·
Franka, in thinking about this, the fuel vaporizing rationale for the "power grooves" might be more of an issue for carburated engines than for the finer and more readily vaporized fuel injected mixtures.
 
#95 · (Edited)
My thoughts on grooved valves are ...its a nice theory and maybe good on a small, long throw crank engines where rpms are low and fuel economy is the priority and therefore there is time in each cycle for the theory to work.

But otherwise velocity and volume rule.

If grooved valves did work we would see it used in some quantity but some auto mfgr. But like I said, I'm not aware of them being used by anyone and/or anywhere.

Regarding carb vs FI power differences...there is no significant difference in high power, high rpm engines.
 
#97 · (Edited)
#98 ·
Alex...I assume those photos answered your question about tulip vs penny-stick valves and back-cut. If not please ask away.
 
#106 ·
Yes, they sure did, thank you!

Hah! He's about 20 minutes away from me. I was thinking of driving over just to see what the project looked like.
If you were referring to my project, you are welcome to stop by. You are in Berkley, CA, right?

Hi Alex...Are you aware of these folks. You can exchange your heads for ported and polished ones. Still stock but flow more and make more torque and hp. http://www.bavengine.com/bavarian_cnc.html

Seriously, I would do this if I had my heads off.
I was not... But now I am :) and it sounds very tempting. I think I'm going to have to give them a call and see what happens.

As for the project, I did a lot of cleaning today again in the engine bay, everything is clean except the engine, which is something for tomorrow. I loooove clean parts :)

I took my AC pump apart the other day (in my other thread) and found that a reed valve broke off (a design flaw apparently), and caused the knocking that started this whole project (I thought the engine was knocking). Bought a replacement pump on eBay, which is the updated model where the design flaw was fixed.
 
#101 · (Edited)
I thought I recalled that 'Metric Mechanic' uses the groove and surface turbulence ST concept and wanted to check it out before posting and commenting.

http://www.metricmechanic.com/catalog/surface-turbulance.php

Their 'grooves' are actually little steps going up the top side of the valve head. MM is a reputable company that has been in the BMW improved performance business for many years.

But still, grooved valves are not used by any auto company, racing team or mentioned in any engine magazines of which I am aware. And I have a subscription to an expensive, foreign magazine that covers all technical aspects of top line racing engines from around the world. Including all the different classes and types.
 
#103 ·
I thought I recalled that 'Metric Mechanic' uses the groove and surface turbulence ST concept and wanted to check it out before posting and commenting.

http://www.metricmechanic.com/catalog/surface-turbulance.php

Their 'grooves' are actually little steps going up the top side of the valve head. MM is a reputable company that has been in the BMW improved performance business for many years.

But still, grooved valves are not used by any auto company, racing team or mentioned in any engine magazines that I am aware of. And I have a subscription to an expensive, foreign magazine that covers nothing but top line racing engines from around the world.

For the sake of truth and completeness I make the MM info available. :angel:
Interesting link. The diagram reminds me of the episode of "Mythbusters," when they showed that if you're driving a pickup truck with an empty bed, you get better gas mileage with the tailgate up than you do with the tailgate down. They measured the fuel consumption both ways, and then they demonstrated the aerodynamics with a scale model in a wind tunnel. With the tailgate up, it creates a "bubble" of air that deflects the main flow so that it passes smoothly over the bed. You couldn't see the effect very well until they showed it using the super high-speed camera.

In any event. Metric Mechanics apparently doesn't claim any fuel vaporization advantage, unlike the ALLPAR article.
 
#102 ·
Engine Masters on news stands now

The latest edition of 'Engine Masters' is currently on the new stands.

I saw it yesterday. I haven't read it yet, but highly recommend it for us gear heads. Usually they go fast so I suggest you p/u a copy very soon.
 
#104 · (Edited)
Hi Alex...Are you aware of these folks. You can exchange your heads for ported and polished ones. Still stock but flow more and make more torque and hp.

http://www.bavengine.com/bavarian_cnc.html

Seriously, I would do this if I had my heads off.
 
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